In an IEEE 802.16 based radio mobile communication system, handover ranging is basically performed during handover of a Mobile Station (MS) for synchronization with a target Base Station (BS). However, to minimize a handover time, a process for adjusting uplink (UL) synchronization through transmission of a Code Division Multiple Access (CDMA) ranging code may be selectively omitted and a ranging request (RNG-REQ) message may be directly transmitted. This is called fast ranging.
FIG. 1 is a flow chart of a general handover process using fast ranging.
First, an MS transmits a handover request (MOB_MSHO-REQ) message to a Serving Base Station (SBS) (step S110). The SBS transmits a handover request (HO-REQ) to a Target Base Station (TBS) (step S120) and receives a handover response (HO-RSP) from the TBS (step S130).
The SBS informs the MS of a time point, at which a fast ranging information element (Fast_Ranging_IE) of TBSs is transmitted, through action time information of a handover response (MOB_BSHO-RSP) (step S140).
If a time which is not 0 is allocated through an action time field, the MS may perform fast ranging to a TBS after that time. The handover response message may include not only an action time but also information about neighboring BSs which can be handed over.
The MS prepares/determines handover to the TBS using information included in the handover response message and transmits a handover indication (MOB_HO-IND) message to the SBS (step S150).
Next, the MS successfully receives the fast ranging information element (Fast_Ranging_IE) from the TBS at a time point indicated by the action time information to receive UL allocation for transmission of a ranging request (RNG-REQ) message (step S160).
The MS transmits the RNG-REQ message to the TBS (step S170) and receives a ranging response (RNG-RSP) message (step S180) to complete authentication. Then, the MS performs user data communication with the TBS.
Such a fast ranging procedure is carried out while the MS receives necessary ranging parameters through a previous scanning process and a reception process of a neighbor advertisement (MOB_NBR-ADV) message.
To prevent loss of a voice or data packet which may be generated during communication failure caused by handover, the MS, SBS, and TBS perform buffering for UL and/or downlink (DL) data. Thereafter, if handover has been successfully performed, corresponding entities transmit the buffered information and thus normal communication can be continuously performed without losing data. The MS may start such buffering from a time point at which a handover indication message is transmitted to the SBS and the SBS may start buffering from a time point at which the handover indication message is received from the MS.
The above-described handover process is described in terms of a delay time with reference to FIG. 2.
FIG. 2 is a diagram showing a general fast handover procedure in a frame unit in a time axis.
In FIG. 2, each message is transmitted in units of 5 ms and it is assumed that a processing time is one Transmission Time Interval (TTI) (2 ms).
First, an MS may receive a DL synchronization frame in a K-th frame to establish synchronization with a TBS (step S201).
The MS may receive a fast ranging Information Element (IE), which may be a form of an UL MAP, from the TBS at a time point indicated by action time information (step S202).
The MS may transmit a ranging request message using UL MAP information to the TBS and the TBS may transmit a ranging response message to the MS (steps S203 and S204).
The TBS may allocate a Channel Quality Indication Channel (CQICH) to the MS and the MS may transmit a Channel Quality Indicator (CQI) code corresponding to channel quality to the TBS through the allocated channel (steps S205 and S206).
Next, the TBS may allocate a UL resource through a DL MAP to the MS and the MS may transmit data through the allocated UL resource (steps S207 and S208).
According to the initial assumption, a total of 40 ms is delayed through the above-described process.
In a radio communication network conforming to general IEEE 802.16e standard, an MS performing handover informs an SBS of information about a BS to which the MS determines to perform handover through a handover indication message. The BS transmits a handover response message to the MS and next transmits UL grant information for transmission of the handover indication message to the MS at an implicit time point or at a time point at which a handover waiting timer has expired. In this way, in order to transmit the handover indication message, the MS should wait for the UL grant information from the SBS. However, since the MS is not aware of a time point at which the UL grant information is transmitted, the MS may encounter the following problems.
First, the MS cannot accurately know until when information can be exchanged with the SBS. Then it is difficult for the MS to perform efficient handover execution scheduling.
Second, if the MS has not received the UL grant information due to a poor channel state, the MS cannot receive information any longer from the SBS. Hence, an unnecessary handover interruption time may be added.
Third, if the MS has not received the UL grant information, since the MS cannot transmit a handover indication message to the TBS, a handover procedure is delayed. Consequently, an unnecessary handover interruption time is additionally generated.